Diesel engines are efficient, durable and economical. In the past 20 years, governments such as the United States and the European Union, have proposed stricter diesel exhaust emission regulations. These environmental regulations require diesel engines to meet increasingly stricter pollution emission standards. Typically, to meet such regulations and standards, diesel engine systems require equipment additions and modifications.
For example, a lean burning engine provides improved fuel efficiency by operating with an amount of oxygen in excess of the amount necessary for complete combustion of the fuel. Such engines are said to run “lean” or on a “lean mixture.” However, the increase in fuel efficiency is offset by the creation of undesirable pollution emissions in the form of nitrogen oxides (NOx). Nitrogen oxide emissions are regulated through regular emission testing requirements. One method used to reduce NOx emissions from lean burn internal combustion engines is known as selective catalytic reduction (SCR). When used to reduce NOx emissions from a diesel engine, selective catalytic reduction involves injecting atomized urea or ammonia gas into the exhaust stream of the engine prior to the catalyst.
The main components of a common SCR system are a tank for storing urea and an injection system for delivering the urea into the exhaust, as well as an SCR catalytic chamber for the NOx reduction to occur. Typically, an onboard diagnostic system monitors the urea level and indicates when the storage tank needs to be refilled. However, while the urea is a high-purity, colorless solution containing 32.5% urea, its high water content (over 65% deionized water) makes this solution very susceptible to freezing in the tank at cold ambient temperatures, as well as very difficult to vaporize in cool exhaust streams.
The fluid cannot be pumped and subsequently sprayed if it is frozen in the tank. Further, when exhaust temperatures are low, poor atomization of the urea can occur and potentially lead to formation of undesirable deposits or the low exhaust temperatures result in a catalyst temperature that is not adequate to achieve the reduction in NOx required to meet the regulations. Accordingly, in order to be in compliance with emission standards, alternate dosing is needed during such times. The present system and methods solve these and other problems associated with prior art NOx reduction systems.